In integration tests, you might want to use helpers from engines that
you mounted in your application. It's not hard to add it by yourself,
but it's unneeded boilerplate. mounted_helpers are now included by
default. That means that given engine mounted like:
mount Foo::Engine => "/foo", :as => "foo"
you will be able to use paths from this engine in tests this way:
foo.root_path #=> "/foo"
(closes#6573)